The present invention relates to an aluminum alloy containing vanadium characterized by improved corrosion resistance and articles made therefrom wherein said articles when anodized have a uniformly grey, light-fast surface and a reflectivity of at most 50%.
The alloy of the present invention consists essentially of 1.20 to 1.60 wt. % iron; 0.25 to 0.55 wt. % manganese; 0.05 to 0.25 wt. % vanadium; up to 0.20 wt. % silicon; up to 0.30 wt. % copper; up to 5.0 wt. % magnesium; up to 0.10 wt. % chromium; up to 2.0 wt. % zinc; up to 0.25 wt. % zirconium; up to 0.10 wt. % titanium; up to 0.50 wt. % total impurities; and the balance aluminum. The present invention includes a method for producing an aluminum article from the alloy set forth above having the characteristics mentioned hereinabove.
Heretofore, various processes are known for achieving a decorative grey color tone on aluminum alloy products. These processes are based on the anodic oxidation of the surface of the aluminum alloy products and do not require additional absorptive coloring. The quality of the resultant color tone and its characteristics are determined by a number of process parameters including particularly the composition of the electrolyte, the voltage applied, the type of electrical current, density and duration and the composition of the particular alloy being employed.
It is common in the prior art to employ two-stage electrocoloring processes and many of these processes are known in the prior art. Classically, in the first stage of the two-stage coloring process, an oxide layer of about 20 .mu.m thick is produced in a sulfuric acid or a sulfuric acid/oxalic acid electrolyte using direct current having a current density of 100 to 200 A/m.sup.2. Following the first stage oxidation, the second stage employs alternating current at a current density of between 10 to 100 A/m.sup.2 in a metal salt solution of desired composition. During the second stage, the metal compounds are precipitated out of the metal salt solution and deposited on the oxide layer such that the metal compounds adhere to the base of the portion in the oxide layer thus forming a permanent light-fast coloring of the oxide.
In addition to the multi-stage coloring processes of the prior art as noted above, a further group of processes for producing light-fast grey tone finishes employ a single stage color anodizing wherein direct current at a current density of 70 to 800 A/m.sup.2 is applied in a special electrolyte to produce oxide layers of natural self-color tone. The color tone obtained in this single stage color anodizing process is dependent on the composition of the alloy and on the electrolyte which comprises organic acids and, if desired, additions of sulfuric acids. Typical aluminum alloys used in this process are aluminum alloys of the type aluminum-manganese, aluminum-magnesium and aluminum-magnesium-silicon alloys.
In addition to the foregoing processes, by employing selected alloys and special processing procedures in the production of semi-finished aluminum articles, it is possible to obtain decorative grey tones on the products with standard anodizing processes. These widely known standard anodizing processes which are very cost attractive, employ direct current at a current density of 80 to 300 A/m.sup.2 and make use of a sulfuric acid electrolyte which often contains additions of carbonic acid. To date, aluminum alloys selected for these anodizing processes contain 4.5 wt. % silicon and 0.5 wt. % magnesium. By using a current density of 150 A/m.sup.2 when anodizing the foregoing aluminum alloy, one obtains after 40 minutes of treatment an oxide layer which is about 18 .mu.m thick and exhibits a moderately grey color tone. The light reflectivity, as a measure of grey tone, amounts to 20%. After an oxidation time of 60 minutes, the oxide layer is 27 .mu.m thick and exhibits a dark grey, self-color finish having a light reflectivity of 13%. The light reflectivity is measured in each case using a LANGE UME 1-LFE 1-measuring instrument.
It has been found that the foregoing aluminum alloy when used in the production of semi-finished products tends to excessively wear the shaping tools used in the production of the semi-finished article. In addition, it has been found difficult to maintain close tolerances in terms of color tone and uniformity.
Accordingly, it is the principal object of the present invention to develop a corrosion resistant aluminum alloy for the production of aluminum products.
It is a particular object of the present invention to develop anodized aluminum articles from said alloy wherein the surface reflectivity of the alloy is uniform.
It is a further object of the present invention to provide a method for producing an improved aluminum article having superior surface qualities than that obtained using conventional processes without the need of additional coloring steps.
Further objects and advantages of the present invention will appear hereinbelow.